Many products use film layers to modify surface characteristics. Polycarbonate ophthalmic lenses, for example, use a film hardcoat layer to protect against scratching and chemical attack. The thicknesses of films used in different applications can range from 0.0001 micron (less than an atom thick) to several hundreds of microns. It is usually important to control the thickness of films used, whether to optimize the performance of the film, or simply to minimize the amount of film precursor that is used.
The most common method of measuring the thickness of non-opaque films less than 100 microns thick is spectral reflectance. Spectral reflectance methods first acquire a range of wavelengths of light reflected off the film structure, and then analyze this reflectance spectrum to determine the film thickness (and often other properties). See for example “Taking the Mystery Out of Thin-Film Measurement,” or, “Spectroscopic Ellipsometry and Reflectometry: A User's Guide” by Tompkins and McGahan, John Wiley & Sons, 1999.Companies such as Filmetrics, Inc. of San Diego, Calif. manufacture such spectral reflectance systems.
Accurate determination of film thickness requires acquiring reflectance spectra that are an accurate representation of the film structure, i.e., the reflectance spectra must be significantly free from contributions from the measuring apparatus. The light reflected off of the film is generally measured using a spectrometer. The amount of light measured at each wavelength is a product of the light source, the film structure, the spectrometer, and the various intermediate optical components used to direct and collect the light. To determine the reflectance spectrum of the film structure, the contributions of the other system components are determined by substituting a known reflectance standard for the film structure, and using the resulting reflectance signal to normalize the subsequent film structure measurements.
Because the reference reflectance standard can not be taken simultaneously with the film structure reflectance, substantial drift in the normalization can occur over time, and this leads to degradation of the quality of the film structure reflectance spectra and, subsequently, the thickness measurement accuracy. Configurations as described above, where the reference is taken before and/or after the film reflectance measurement, are known as “single-beam” reflectance systems. In practice, references taken with single-beam configurations take operator intervention, which means that the time interval between references is generally large and that errors are possible. The result is that operator time is consumed and measurement accuracy degrades.
The primary alternative to the single-beam configuration is the dual-beam configuration, which splits off a portion of the light source and routes it to a second spectrometer. This allows for real-time monitoring of and correction for light source drift. However, dual-beam configurations are almost twice as expensive as single-beam configurations (since the spectrometer is usually the most expensive system component) and they do not take into account spectrometer drift or that of most of the optical path. An excellent review of single-versus dual-beam configurations, as well as patent literature relevant to this application, is included in U.S. Pat. No. 6,831,740 for example. Consequently, there is a need for systems and methods that provide an intermediate in-line nearly real-time reference for single-beam reflectance configurations.